As shown in FIG. 15 of the accompanying drawings, a fuel tank 200 for a motorcycle generally has an outer side panel 202 and an inner side panel 204 having its bottom welded to the outer side panel 202. The outer side panel 202 and the inner side panel 204 have respective ends bent downwardly into flanges 206 that are generally seam-welded.
In motorcycles, a motorcycle having a steering handle at a high position, and which is operated by a rider whose upper half body is kept substantially upright, i.e., a so-called American-type motorcycle, has its fuel tank 200 regarded as being particularly important in terms of appearance. For the fuel tank 200 to be aesthetically pleasing, the welded flanges 206 should preferably not be exposed. Furthermore, since the center of gravity of the fuel tank is displaced upwardly by the height of the flanges 206, the flanges 206 are disadvantageous with respect to efforts to lower the center of gravity of the motorcycle.
The existence of the flanges 206 also limits the capacity of the fuel tank 200.
There has been proposed a fuel tank having a structure in which flanges do not project downwardly due to seam welding, but rather are bent inwardly (see, for example, Japanese Laid-Open Patent Publication No. 10-76985). According to this proposal, however, useless space is present above the flanges, which limits the capacity of the fuel tank.
For manufacturing a flangeless fuel tank, a skilled welder is required, in order to weld the fuel tank by means of arc welding or the like. If the fuel tank is automatically welded by a robot, then since the fuel tank must be firmly secured in place, the fuel tank tends to crack since thermal strains cannot be relieved during welding, resulting in a reduced yield. In such a case, when the fuel tank cracks, it still needs to be repaired by a skilled welding operator.